chore(nix): Update `nim` version to `1.6.18` in the dev env

flake.lock

@@ -51,10 +51,27 @@
         "type": "github"
       }
     },
+    "nixpkgs-unstable": {
+      "locked": {
+        "lastModified": 1705316053,
+        "narHash": "sha256-J2Ey5mPFT8gdfL2XC0JTZvKaBw/b2pnyudEXFvl+dQM=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "c3e128f3c0ecc1fb04aef9f72b3dcc2f6cecf370",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixos-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
     "root": {
       "inputs": {
         "flake-parts": "flake-parts",
-        "nixpkgs": "nixpkgs"
+        "nixpkgs": "nixpkgs",
+        "nixpkgs-unstable": "nixpkgs-unstable"
       }
     }
   },

flake.nix

@@ -1,11 +1,13 @@
 {
   inputs = {
     nixpkgs.url = "github:NixOS/nixpkgs/nixos-23.11";
+    nixpkgs-unstable.url = "github:NixOS/nixpkgs/nixos-unstable";
   };
 
   outputs = inputs @ {
     nixpkgs,
     flake-parts,
+    nixpkgs-unstable,
     ...
   }:
     flake-parts.lib.mkFlake {inherit inputs;} {
@@ -19,7 +21,7 @@
         devShells.default = with pkgs;
           mkShell {
             packages = [
-              nim
+              nixpkgs-unstable.legacyPackages.x86_64-linux.nim1
               nimPackages.nimble
 
               # Deps for waku